1. Field of the Invention
The present invention is generally related to subscription entertainment systems such as pay-for-view television and more specifically to methods and systems for scrambling and descrambling entertainment information in a manner which substantially prevents receivers from obtaining entertainment value from transmissions which they are not authorized to enjoy.
2. Description of the Relevant Art
A method for scrambling the video content of a television signal is disclosed in U.S. Pat. No. 4,716,588, issued Dec. 29, 1987, to Thompson et al. under the title "Addressable Subscription Television System Having Multiple Scrambling Modes". The disclosure of this previous patent (Thompson '588) is incorporated herein by reference and its detailed disclosure will not be repeated here.
According to the Thompson '588 patent, video entertainment information may be transmitted in one of three modes. It can be transmitted either in (1) a straightforward nonscrambled mode (i.e., standard NTSC format), (2) a first scrambled mode wherein the video signal content of all odd-numbered display lines in each frame are inverted while remaining lines are noninverted, or (3) a second scrambled mode wherein the video signal content of all even-numbered display lines in each frame are inverted while remaining display lines are noninverted. Descramble instructions are sent to a descrambling receiver unit at a subscriber location for switching the descrambling unit among three corresponding and complementary receiving modes, namely, (1) forwarding received video signals straight through to a standard television set without additional processing, (2) descrambling a received video signal by inverting the video content of all odd-numbered video display lines in each frame while leaving remaining display lines of the frame in their received form and (3) descrambling the video content of a received signal by inverting all even-numbered display lines in each frame while leaving remaining display lines of the frame in their received form. The descramble instructions are digitally represented by pulses in the top 20 horizontal lines of each video frame. These top 20 horizontal lines constitute the nonviewable vertical blanking interval of a standard television display frame and accordingly, the typical viewer is left unaware of the presence of the descramble instructions.
While this technique may prevent novice viewers from obtaining unauthorized entertainment value out of transmissions, it is not difficult for sophisticated viewers to determine on a frame by frame basis whether or not line inversion is taking place and to further determine by guessing, whether inversion begins at the first odd or first even numbered display line of each frame. Because frames are presented at a relatively slow rate (i.e., 60 frames per second), and because the audio portion of a television show is not scrambled, it can become relatively simple for sophisticated viewers to construct unauthorized devices for descrambling television signals which they are not authorized to enjoy. A more secure method for transmitting entertainment information and permitting only authorized viewers/listeners to obtain enjoyment from the same is needed in the industry.
Numerous alternate techniques have been proposed for preventing unauthorized program enjoyment. Included among these are a line chopping technique disclosed in U.S. Pat. No. 4,396,947, issued Aug. 2, 1983, to Chung, "Apparatus For Encoding of Information" and a so-called "line-cut and rotate" method proposed by Thompson S. A. and NewsData Security Products Ltd. These proposed methods suffer from design complexity and unreliability, especially when noise is present in the signal transmission channel.